1. Field of the Invention
This invention relates to a lens system comprised of a single component aspherical lens and an aperture stop, and, more particularly, to a taking lens system comprised of an aspherical lens and an aperture stop suitable for easy-to-use photographic cameras and closed circuit TV cameras.
2. Description of Related Art
Typically, easy-to-use or disposable cameras, such as what is called a "lens equipped film package," have single component taking lenses with diaphragms or aperture stops. Although such a single component taking lens system is usually inferior in optical performance to multiple component taking lens systems, nevertheless it is still sufficiently practical for easy-to-use cameras and really simplifies the structure of a camera.
Single component lenses for use with single component taking lenses are mostly of the type having one surface convex to the subject end as known from, for instance, Japanese Unexamined Patent publications No. 63-199,313 and No. 63-106,710. However, because such a single component lens is insufficient in correction of aberrations, in particular chromatic aberrations or chromatic aberrations of magnification, it must be improved in order to provide desirable resolution.
A reference is made to FIGS. 1 through 3 for the purpose of providing a brief introduction of a conventional single component taking lens systems that will enhance an understanding of the optical structure of the single component taking lens system of the present invention.
As shown in FIG. 1, a taking lens system is comprised of a single component lens 1 and a fixed aperture stop 2. The single component lens 1 is a meniscus lens provided so that it has a convex surface facing to the subject end. It is featured in the prior art taking lens system that the fixed aperture stop 2 is attached to the single component lens 1 on the image end. For this taking lens system, the correction of achromatic aberrations is insufficient.
Further, when applying single component taking lens systems to what are called "Z-type easy-to-use cameras," in which an optical path is folded in a "Z" form, complications arise because of a relatively short back focal distance. For instance, referring to FIG. 2, which schematically shows an optical arrangement of a Z-type easy-to-use camera, a single component taking lens 1a, comprised of a biconvex single component lens, has a convex surface facing to the subject end. Behind the single taking lens 1a, or on the image end of the taking lens 1a, there are provided a stationary aperture stop 2a, a shutter 3a and reflection mirrors 4 and 5 arranged in an optical axis Xa in order from the object end toward the image end. The reflection mirrors 4 and 5 are placed in parallel with each other so as to form a parallel folded optical axis Xa between the single component taking lens 1a and an image plane 15a in which a film is placed.
In such a Z-type easy-to-use camera with a single component taking lens 1a whose convex surface is directed to the object end, since a back focus distance, namely a back focal distance, of the single component taking lens 1a is unavoidably relatively short, the camera components, i.e. the stationary aperture stop 2a, shutter 3a and reflection mirrors 4 and 5, have to be disposed and laid out in a short axial distance between the single component taking lens 1a and the image plane 15a. Such a tight disposition of the camera components renders the camera components permissive to interfere with the optical path.
In an attempt to avoid the issue of mechanical lay out, it is acceptable to employ a single component taking lens with its surface concave to the object end. This is because such a single component lens 1b, with its concave surface directed toward the object end, is provided with an aperture stop 2b placed on the object end or an opposite side to an image plane 15b as shown in FIG. 3. Such is described in, for instance, Japanese Unexamined Patent Publication No.64-49014. However, the single component lens, such as a meniscus lens, renders camera components still permissive to interfere with the optical path.